Millions of people in the Chinese city of Lanzhou scrambled to buy bottled water this weekend after the city’s water supply was contaminated with Benzene. Levels of the cancer-causing chemical in the city’s tap water were discovered to be 20 times China’s national limit.

All of this comes as China is coming to grips with the environmental damage caused by decades of unprecedented growth. Lanzhou officials are blaming two explosions – one of them 27 years ago, the other 12 years ago – at oil refineries in the area.

They say these explosions caused oil to slowly seep into the groundwater, and that this sudden rise in levels of Benzene shows the decades-old oil is now contaminating the city’s water supply.

“Nearly 80% of chemical industry is built in densely populated city areas,” said Du Sha of Greenpeace China, “So this type of data shows that currently the chemical industry raises the high risk to the public health. The government should take more prevention and more supervised measures to manage these chemical industries.”

Many residents of Lanzhou say the local government should have informed the public much earlier than they did about the water contamination. The state media is now reporting that city officials waited nine days to tell the public that their water was contaminated. The Lanzhou government now says water quality in the city is returning back to normal.